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Dual Radio Frequency Observations of Swift J1818.0-1607 with the Deep Space Network

ATel #13649; Walid A. Majid (JPL, Caltech), Aaron B. Pearlman (Caltech), Thomas A. Prince (Caltech, JPL), Charles J. Naudet (JPL), Jonathon Kocz (Caltech), Shinji Horiuchi (CSIRO, CDSCC), Teruaki Enoto (RIKEN), George Younes (George Washington University)
on 21 Apr 2020; 00:33 UT
Credential Certification: Aaron B. Pearlman (aaron.b.pearlman@caltech.edu)

Subjects: Radio, X-ray, Neutron Star, Soft Gamma-ray Repeater, Star, Transient, Pulsar, Magnetar

Referred to by ATel #: 13898, 13966, 14001

Swift J1818.0-1606 is a newly discovered pulsar, initially detected with Swift's BAT instrument when a short X-ray burst was observed on 2020 March 12 (GCN circular #27373). A 1.36 s period was discovered in a follow-up observation with NICER (ATel #13551), which suggested that the source is a new magnetar. A series of subsequent radio observations, carried out at several wavelengths, have confirmed the periodic emission (ATel #13553; ATel #13554; ATel #13559; ATel #13560; ATel #13562; ATel #13575; ATel #13577; ATel #13580; ATel #13587; Esposito et al. 2020). The pulsar's rotational period derivative of 8.16(2) x 10^-11 implies a surface dipolar magnetic field of 3.4 x 10^14 G and a characteristic age of 265 years (ATel #13559). Further timing observations with NICER yielded a spin-down luminosity of 1.1 x 10^36 erg s^-1 (ATel #13588), close to that of PSR J1119-6127, a high magnetic field rotation-powered pulsar.

We carried out radio observations of Swift J1818.0-1607 using the Deep Space Network (DSN) 70 m diameter radio telescope (DSS-63) in Madrid, Spain at center frequencies of 2.3 GHz (S-band) and 8.4 GHz (X-band). The DSN observations were performed simultaneously at the two frequency bands on 2020 March 26, starting at 08:30:00 UTC, for 7440 s (Epoch 1) and on 2020 April 8, starting at 07:37:54 UTC, for 6840 s (Epoch 2). Dual circular polarization data were recorded with bandwidths of ~110 MHz at S-band and ~396 MHz at X-band using the pulsar backend in filterbank search mode with a frequency and time resolution of 0.46 MHz and 2.2 ms, respectively.

We searched both data sets for evidence of pulsed emission near the spin period of the pulsar after dedispersing the data using a dispersion measure (DM) value of 706.0 pc cm^-3. In Epoch 1, we detected strong pulsed emission at S-band after folding the data with a barycentric spin frequency of 0.7333684(2) Hz, which was derived from fitting a phase-coherent timing model (assuming a constant rotational frequency) to pulse time of arrivals (ToAs) derived from sub-integrations across the observation duration. The mean flux density at S-band was 0.8(2) mJy. We place a 7-sigma upper limit of < 0.03 mJy on the flux density at 8.4 GHz during Epoch 1, assuming a 10% duty cycle, since no evidence of pulsed emission was detected at X-band. Assuming a power-law dependence on flux density with observing frequency, we place an upper limit of < -2.2 on the spectral index during Epoch 1.

We carried out a similar analysis using simultaneous S-band and X-band data during Epoch 2 and detected pulsed emission at both observing frequencies after folding the data with a barycentric spin frequency of 0.73335175(6) Hz. The mean flux densities at S-band and X-band were 0.31(6) mJy and 0.026(5) mJy, respectively. To our knowledge, this is the first reported flux density measurement of Swift J1818.0-1607 at or near 8 GHz. Based on these flux density measurements, we obtain a spectral index of -1.9(2) between 2.3 and 8.4 GHz during our Epoch 2 observations. This spectral index value is consistent with the average spectral index of -1.8(2) associated with the majority of pulsars (Maron et al. 2000). It appears that the pulsar's radio emission is evolving based on these recent measurements, which is quite typical of high magnetic field pulsars and radio magnetars. Pulse profiles showing the evolution of the radio emission during Epoch 1 and Epoch 2 are provided at the bottom of this ATel.

Additional high frequency radio observations will be particularly informative given the strong, dynamic radio emission typical of magnetars at high frequencies.

We thank the DSN team for their rapid response in scheduling the observations.

Pulse Profiles of Swift J1818.0-1607 (Deep Space Network, DSS-63, 2.3 and 8.4 GHz, 2020 March 26 and 2020 April 8)